In recent memory hard disk drives, high storage capacity and miniaturization have been demanded. In order to increase the recording density, it has been strongly urged to lower flying height of a magnetic head and to reduce the unit recording area. Along with this trend, the surface quality required after polishing has become severely assessed every year even in a process for manufacturing a substrate for memory hard disk (hereinafter referred to as “substrate for a disk”), so that the surface roughness and the sizes and depths of waviness allowed have been becoming increasingly small.
In order to meet such requirements, there has been proposed a polishing composition containing silica particles as abrasive particles having a specified particle size distribution (see, for instance, Japanese Patent Laid-Open No. 2002-30274).
It is disclosed in the publication that according to the above-mentioned polishing composition, since colloidal silica particles having different mono-modal number particle size distributions are contained, an aluminum disk substrate having small average waviness and a few surface defects can be obtained.
In addition, there has been known a polishing composition containing colloidal silicas having a specified particle size distribution, which has excellent surface smoothness, whereby polishing can be carried out at an economical rate without generating surface defects (see, for instance, Japanese Patent Laid-Open No. 2001-323254).
However, there are some cases in the polishing composition where squeaky noises or vibrations from the surroundings of the holder (carrier) of the substrate for a disk are generated when the substrate for a disk is polished using a polishing machine (hereinafter this phenomenon being referred to as “carrier squeals”). Therefore, the surface defects (scratches) have been rather increased, thereby leading to lowering of the surface smoothness of the substrate for a disk.
In addition, in the polishing composition, there has not been any contrivances in a case where alumina abrasive grains are used as primary polishing, from the viewpoint of preventing defects of pits which are caused by the alumina abrasive grains remaining on the substrate surface (hereinafter referred to “micropits”). Therefore, further improvements have been desired.
The term “micropits” as referred to herein is surface defects which differ from usual pits in that the micropits cannot be clearly detected as a bright spot with a differential interference optical microscope, so that the defects have not conventionally been satisfactorily studied. Therefore, the micropits have been recently rapidly recognized as surface defects for substrates which hinder the miniaturization of the unit recording area.